Two suggestions:

Placenta previa, leading to placental abruption. More info here and here. It can cause complications with the baby as well, but often is only a serious risk to the mother. Abruption can (rarely) lead to a fatal hemorrhage.

Preeclampsia/eclampsia is a disorder (its causes are not well understood) primarily characterized by rising blood pressure and high protein levels in the mother's urine. If it progresses to eclampsia, death of the mother has been known to occur. More info here.

Do you know the most surprising thing about divorce? It doesn't actually kill you, like a bullet to the heart or a head-on car wreck. It should. When someone you've promised to cherish till death do you part says, "I never loved you," it should kill you instantly.

- Under the Tuscan Sun